MAKES MIGRATING BIRDS KEEP FLYING?
is your vocabulary word for today: ghrelin. Ghrelin is an amino-acid
peptide hormone that affects appetite and the secretion of growth hormone
in various organisms.
findings demonstrate that, for migrating birds, ghrelin governs when
a bird will continue its trip after stopping en route between northern
and southern destinations. People have known about bird migration for
centuries, but knowing what causes an individual bird to decide when
to resume its long journey after a pit stop along the way has eluded
ornithologists until now.
to a study published in the Proceedings of the National Academy of
Sciences, Twice a year, billions of birds migrate across continents.
We see such migrations in the United States every spring and fall with
hummingbirds, swallows, waterfowl and other avian species.
explanation for migration is that birds move to warmer feeding areas
during autumn to avoid winter cold and depleted food sources. They return
to cooler areas in spring and summer to breed when food has become more
factor influencing the onset of migration is the increasing period of
daylight in spring and vice versa in the fall. But flying takes calories,
so during a stopover, how does a bird know it has acquired enough fuel
to continue its journey? A study conducted by Wolfgang Goymann (Max
Planck Institute for Ornithology in Germany) and several colleagues
may have found the answer.
studied garden warblers, which migrate across the Mediterranean Sea
between northern Europe and central Africa. On the return trip, many
of the little birds land on Ponza, a small island off the western coast
of Italy. The Mediterranean is much larger than the Gulf of Mexico,
across which some U.S. birds migrate, and Ponza is used as a feeding
stopover before the birds reach the mainland and continue north. After
such a long flight, it is essential for them to linger and refuel. But
at some point they must resume their travel. The researchers wanted
to find out what prompts them to leave.
measurements of body fat of arriving birds in the spring, the researchers
determined that food intake and the amount of stored energy of a garden
warbler is controlled by its hormone levels. Field experiments revealed
that injections of ghrelin caused birds to decrease their food consumption.
Subsequently, the inclination of a bird to continue migrating increased.
Furthermore, the higher the fat levels in a bird, the higher the concentration
of ghrelin. Researchers found that the hormone controls the amount of
food necessary for flight and is also indicative of a birds fat
levels, a useful tool in research.
team also conducted hormonal experiments and found that birds they injected
with ghrelin decreased their food intake and increased their drive to
continue migration. The interpretation is that a birds migratory
behavior is regulated by the hormonal system that controls the amount
of food consumed and therefore the amount of stored energy required
for long distance flying.
among hormones that affect diet are complex and not yet fully understood,
but mammals and birds alike are affected by the presence or absence
of ghrelin. For example, previous studies found that the ghrelin hormone
stimulated food consumption in rodents but inhibited it in chickens
and quail. The garden warbler study established a connection between
stored food and migratory behavior.
to the research publication, the discovery of the role of ghrelin in
migratory birds reveals a hormonal system shared by birds and
mammals, whose disruption causes eating disorders, obesity and
complications in metabolism. The implications of the hormone ghrelin
for controlling the appetite in people who need some help with that
are extraordinary. More research will be necessary to make the use of
ghrelin a medical reality for diet control, but the initial work on
garden warblers demonstrates the importance of understanding connections
between ecology and humans.
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